If both guitars buzz with light strings but not with heavier strings, then there are a couple of options. I'm assuming the buzz is happening when the string touches a fret somewhere near the middle of its vibrating length; if the buzz is at the nut or bridge or somewhere else, then this line of thinking does not apply.
You could switch to a heavier gauge, or a set with light-tops/heavy-bottoms. Or, you may be able to use your desired set if you raise the action on the bass strings. Bridge-adjustments vary with the style of bridge, but on most recent acoustics I've seen the bridge is a removable piece of bone or plastic. (Do not do this yourself with a piezo pickup system, you may affect the balance of the pickup signal.) To raise this type of bridge, you
- slacken (or remove) all the strings,
- remove the bridge piece,
- add 1 or 2 paper shims into the cavity where the bridge piece goes,
- replace bridge piece,
- tune-up and evaluate (are we done or does it need another few shims?)